The present invention relates generally to a lock-up type automatic transmission, and particularly to an improvement in a lock-up control whereby occurrence of substantial shift shocks is prevented.
Commonly, automatic transmissions have a torque converter in an engine power delivery path thereof in order to multiply torque from an engine. In the torque converter, an inlet element (a pump impeller) driven by an engine imparts a rotational force to a working hydraulic fluid within the torque converter, and the rotation of the fluid causes an output element (a turbine runner) to rotate under the reaction of a stator, thus multiplying an engine torque (this mode of operation being called as "a converter state"). Owing to this hydrodynamic transmission of power, the torque converter is subjected to a slip between the pump impeller and the turbine runner while it is in operation. Thus, the automatic transmissions having a torque converter in the engine power delivery path thereof is poor in fuel economy due to poor power transmission effeciency although they are easy to operate. In order to alleviate this drawback, there has been conventionally proposed a torque converter with a so-called bridge clutch (which may be called as a lock-up torque converter) wherein the turbine runner is directly and mechanically connected to the pump impeller (this mode of operation being called as "a lock-up state" ) in order to eliminate the slip at relatively high vehicle speed range where the engine is not subject to substantially torque variations. Recently, some automobiles have begun to use such an automatic transmission with a lock-up type torque converter.
Referring to FIG. 1, a shift pattern diagram illustrates lock-up ranges of an automatic transmission wherein the torque converter with the bridge clutch assumes the lock-up state when an actual vehicle speed is higher than a preset vehicle speed value (i.e., a lock-up vehicle speed) for each of forward speed or gears. In this Figure, there is illustrated a shift schedule for upshiftings to take place in a three-speed automatic transmission, where V1, V2 and V3 designate lock-up vehicle speeds for first, second and third speeds, respectively, and A, B and C designate lock-up ranges for the first, second and third speeds, respectively. In the case of automatic transmission where the torque converter locks up when the vehicle speed is higher than the lock-up vehicle speed as mentioned above, a shifting between two adjacent forward speeds with the accelerator pedal depressed deeply by a great degree (with a large throttle opening degree) takes place when the torque converter remains in the lock-up state thereof as will be readily understood from the fact that the lock-up ranges A, B and C are disposed one next to another along the vehicle speed at the large throttle opening degrees. If the shifting were made with the torque converter in the lock-up state therof, substantilly great shocks would take place because the torque variations upon shifting could not be absorbed.
In the lock-up type automatic transmission of this kind, the above mentioned problem has been tried to solved although not completely by releasing the lock-up action upon shifting even during operation in any one of the above mentioned lock-up ranges, thereby to temporarily render the torque converter to operate in the torque converter state thereof. It is therefore the common practice to construct a control arrangement wherein a shift detecting circuit is provided which generates a shift signal indicative of in-shifting state for a predetermined time after a command for shifting has been made and the lock-up action is interrupted temporarily while the shift signal from this circuit is present even during operation in any one of lock-range ranges.
Describing how the above mentioned shift detecting circuit works in shifting from the second speed to the third speed referring to FIG. 8(a), it generates a shift signal at the same instant t1 when a command for shifting takes place for releasing the lock-up (L/u) action for a predetermined time T'. According to this control strategy, the lock-up action is released too early, thus posing inconveniences as follows. There is a time lag from the instant when the command for shifting is made to the instant when the actual shifting operation initiates, viz., initiation of actuation of the friction elements, because of the existence of a response delay in the hydraulic control system of the transmission. Therefore, if the lock-up action is released simultaneously with the instant when the command for shifting is made, the lock-up action is released before the actual shifting operation begins, an engine revolution speed rises rapidly during a moment from t1 to t2 as shown in FIG. 8(a), causing the engine to race. Owing to the fact that releasing of the lock-up terminates so as to allow the lock-up action to resume at the instant during a moment from t3 to t4 when the shifting operation actually takes place, the torque converter cannot aborb shift shocks inherent with the shifting operation, and this fact cooperates with the fact that engine revolution speed has risen resulting from engine racing as mentioned, increases the magnitude of a peak torque at the instant t5 right after the shifting operation, with the inevitable result that substantial shift shocks occur. This tendency becomes marked when the automatic transmission is subject to an upshifting because the upshifting takes place with the power-on mode. This problem is more serious upon upshifting from the second speed to the third speed where a front clutch that is to be engaged for the third speed is engaged while releasing a second brake which is to be applied for the second speed, thus requiring a long so-called overlap time, causing the shifting operation for this upshifting to be delayed as compared to the other shifting operations.
As one measure to solve this problem, it is conceivable to elongate the lock-up interrupt time T' up to the instant when the shifting operation will be completed, but this leaves the racing problem of the engine unsolved. The increase in the engine revolution speed causes the corresponding increase in the magnitude of shift shocks.
For solving the above mentioned problem, U.S. Pat. No. 4,431,095 issued to Massaki Suga on Feb. 14, 1984 has disclosed a lock-up type automatic transmission wherein a delay circuit is provided so as to delay generation of a shift signal for a predetermined time T1 after the instant t1 when the command for shifting is made as shown in FIG. 8(b), whereby the interruption (OFF) of the lock-up (L/u), which is to take place upon shifting during operation in any one of the lock-up ranges, is rendered to coincide with the instant t3 when the actual shifting operation begins.
However, a delay time from the instant t1 to the instant t3, i.e., a delay from the instant when the command for shifting is made to the instant when the actual shifting operation initiates, varies from one manufacturing product to another due to manufacturing dispersion among products, i.e., a difference in flow resistance in shift control fluid passages and variation in viscosity of working fluid, and it has been confirmed that the predetermined time T1 set by the above mentioned delay circuit does not necessarily agree with the delay time from t1 to t3 with the result that with this conventional measure the above mentioned engine racing and substantial shift shocks cannot be prevented.